Siloxane resins are known to be materials from which films of high heat resistance, of high durability, of high hardness, of low dielectricity, of high insulation and of high transparency can be formed by thermal curing to hardening (hereinafter, often simply referred to as “hardening”), and hence are used for various applications. For example, they are employed for forming insulating films, planarization films or protective films in semiconductor devices and liquid crystal displays, and also adopted as sealants of semiconductor devices. Further, since having high transparency, they are used not only as materials in the field of electronics but also as materials for forming surface protective films of optical elements and automobiles.
In the case where a cured film is intended to be formed from a coating composition comprising a solvent and a siloxane resin serving as a binder, it is known to employ a multifunctional polysiloxane having alkoxy groups or hydroxyl groups (Patent documents 1 and 2). However, as far as the present inventors know, the above multifunctional polysiloxane has room for improvement in view of stability in a solution. Specifically, if the composition has high viscosity or high solid content, that multifunctional polysiloxane can suffer from a problem of easily gelling or getting more viscous immediately after addition of a hardening agent or a catalyst, such as, an acidic compound, a basic compound, a metal alkoxide or a metal chelate compound. Further, even if neither gelling nor getting more viscous immediately after the addition, the multifunctional polysiloxane often experiences an increase in viscosity while being stored.
This problem is presumed to be because the above siloxane resin has alkoxy groups or hydroxyl groups. Accordingly, in order to avoid the problem, it is studied to adopt a siloxane resin having neither alkoxy groups nor hydroxyl groups. However, that siloxane resin often has too low reactivity to be hardened sufficiently by curing. Further, it is also studied to use a siloxane resin in which the alkoxy or hydroxyl groups are replaced with other organic reactive groups. However, that siloxane resin is often unsatisfactory in some respects, such as, thermal stability, electric properties and the like of the resultant film. In addition, it is often disadvantageous in view of the cost to employ a multifunctional polysiloxane whose structure or reactive groups are not typical (Patent documents 3 to 6).